Summary Our overall goal is to investigate developmental processes in visual cortex (V1) that do not require visual experience but are instead influenced by endogenous neuronal activity. This initial experience-independent stage of development is critically important for the formation of rudimentary circuits that are subsequently sculpted by visual experience. Neonatal mammals spend most of their time in rapid-eye-movement (REM) sleep, which suggests that this brain state provides a principal source of endogenous activity to the developing brain. This idea has not been explored in depth using current neurobiological methods. The proposed exploratory research will thus provide important new insights into how sleep and experience together shape developing circuitry and how abnormal sleep during infancy may adversely impact brain development. To achieve our goal, we will measure V1 activity in developing ferrets to determine if spontaneous activity in REM sleep is structured in a manner that may instruct or maintain developing circuits. We will then directly test the role of REM sleep V1 activity in the development of visual response properties. This will be accomplished using a combination of polysomnography, genetically encoded calcium indicator imaging and optogenetics (optimized for small mammals). This A1 proposal has been revised in accordance with recommendations from prior review.